The present invention relates to a transmitting apparatus and a reproducing apparatus whereby a one-bit digital audio signal acquired through delta-sigma modulation is converted to a multi-bit audio signal to be transmitted.
This application has already proposed a system whereby an analog audio signal is subjected to delta-sigma modulation at a sampling frequency significantly higher than that of the conventional compact disc (CD) in order to transmit, record and reproduce a one-bit digital audio signal.
The one-bit digital audio signal above is radically different from a multi-bit digital audio signal exemplified by the compact disc (CD) having a sampling frequency (fs) of 44.1 kHz and 16 quantization bits. That is, a high-quality audio standard offering a wide band and a wide dynamic range is implemented by means of the one-bit digital signal system that involves the quantization bits being sampled at a sampling frequency 64 times the sampling frequency (fs) of the conventional CD. This high-quality audio standard is called the DSD (Direct Stream Digital) standard, and discs having data recorded thereon as per the DSD Standard comply with what is known as the Super Audio CD Standard.
A one-bit digital audio signal obtained by delta-sigma modulation may be converted to a conventional multi-bit digital audio signal through down-sampling by a converter such as a decimation filter. Before the one-bit digital audio signal is output to a conventional digital audio interface that complies with IEC 958 (International Electrotechnical Commission 958), the signal needs to be converted to the conventional multi-bit digital audio signal through down-sampling.
In a setup where an analog audio signal is converted to a one-bit digital audio signal through delta-sigma modulation, there is defined a degree of modulation corresponding to the input conversion level in effect. That is, the modulation degree represents level information that is possessed by the one-bit digital audio signal in question. A maximum modulation degree, i.e., a maximum allowable input level defined by a delta-signal modulator in use, is generally established in accordance with an optimum point of modulator characteristics. If an analog audio signal level is input so as to obtain a maximum modulation degree, the rise of the level seldom results in an abrupt deterioration of level and distortion characteristics specific to an output one-bit digital audio signal.
Meanwhile, the modulation degree of an input one-bit digital audio signal and the level of an output multi-bit digital audio signal are generally determined in a unique manner for each decimation filter. For example, where a decimation filter is designed to have a maximum modulation degree of 50 percent, the filter outputs a multi-bit digital audio signal at its maximum value (i.e., full bit level) when a one-bit digital audio signal with the modulation degree of 50 percent is input. Upon input of a one-bit digital audio signal exceeding the maximum modulation degree, the multi-bit digital audio signal is kept fixed to the maximum value, i.e., brought into a clipped state.
The process above is explained below in more detail with reference to FIG. 1 which shows a conventional digital signal processing apparatus 50. In the digital signal processing apparatus 50, an analog audio signal A1 input through an input terminal 51 is fed to a one-bit delta-sigma modulator 52 for delta-sigma modulation into a one-bit digital audio signal D1. The one-bit digital audio signal D1 from the modulator 52 is supplied to both a one-bit D/A converter 53 and a decimation filter 55.
FIG. 2 illustrates a typical constitution of the one-bit delta-sigma modulator 52 mentioned above. The one-bit delta-sigma modulator 52 comprises an adder 71, an integrator 72, a quantizer 73, and a delay circuit 75. An added output of the adder 71 is fed to the integrator 72. An integrated output of the integrator 72 is sent to the quantizer 73. A quantized output of the quantizer 73 is led out of an output terminal 74 and fed through the delay circuit 75 back to the adder 71 in which the quantized output is added to an analog audio signal coming from an input terminal 70. The added output of the adder 71 is integrated by the integrator 72 whose integrated output is in turn quantized by the quantizer 73 per sampling period. After the processing, one-bit quantized data, represented by the one-bit digital audio signal D1, are output from the output terminal 74.
The one-bit D/A converter 53 converts the one-bit digital audio signal D1 to an analog audio signal A0 that is sent to an output terminal 54. Generally, the audio signal composed of one-bit digital audio signals is converted back to the analog audio signal by the D/A converter as described.
The decimation filter 55 turns the one-bit digital audio signal D1 into a multi-bit digital audio signal DM that is fed to an output terminal 56. The decimation filter 55 converts the one-bit digital audio signal having the sampling frequency of 64xc3x97fs into a multi-bit digital audio signal of 16 bits with the sampling frequency of fs, whereby sound quality equivalent to that of CD is acquired. Typically, the decimation filter 55 has the maximum modulation degree of 50 percent.
There is a problem when the decimation filter 55 having the maximum modulation degree of 50 percent constitutes part of a converter that converts the one-bit digital audio signal D1 through down-sampling into the multi-bit digital audio signal DM. That is, if the level of an input one-bit digital audio signal D1 exceeds the maximum modulation degree, the output multi-bit digital audio signal DM is fixed to a clip level.
Below is a detailed description of the deficiency outlined above. FIG. 3 is a graphic representation of characteristics indicating levels of an analog audio signal A0 output by the one-bit D/A converter 53 and levels of a multi-bit digital audio signal DM furnished by the decimation filter 55 with respect to modulation degrees (input signal levels) of a one-bit digital audio signal D1 from the one-bit delta-sigma modulator 52 in the digital signal processing apparatus 50. The levels of the analog audio signal A0 are plotted by a broken line characteristic 61 while the levels of the multi-bit digital audio signal DM are depicted by a solid line characteristic 62.
The broken line characteristic 61 indicates that the levels of the analog audio signal A0 remain approximately linear when the modulation degree of the input signal exceeds 50 percent. This is what is known as a soft clipped state, not perfectly linear, but almost. On the other hand, as indicated by the solid line characteristic 62, the multi-bit digital audio signal DM is clipped to a full bit level when the modulation degree exceeds 50 percent. The decimation filter 55 is designed to effect output at the full bit level illustratively when the modulation degree reaches 50 percent. Thus an input with the modulation degree of 50 percent or higher causes the output signal to be clipped as exemplified by the characteristic 62.
FIG. 4 is another graphic representation of characteristics plotting distortion factors of the analog audio signal A0 output by the one-bit D/A converter 53 as well as distortion factors of the multi-bit digital audio signal DM furnished by the decimation filter 55 with respect to modulation degrees of the one-bit digital audio signal D1 in the digital signal processing apparatus 50. The distortion factors of the analog audio signal A0 are denoted by a broken line characteristic 63 while the distortion factors of the multi-bit digital audio signal DM are represented by a solid line characteristic 64.
The broken line characteristic 63 indicates that the distortion factor of the analog audio signal A0 is at a minimum point when the modulation degree is about 50 percent. This means that the best characteristic is obtained for the analog audio signal A0 when the one-bit delta-sigma modulator 52 is operating with its distortion factor at about 50 percent. On the other hand, as indicated by the solid line characteristic 64, the distortion factor of the multi-bit digital audio signal DM from the decimation filter 55 leaps abruptly when the modulation factor is near 50 percent. This phenomenon is attributable to the fact that the multi-bit digital audio signal DM is clipped to the full bit level when the modulation factor comes close to 50 percent.
As can be judged from FIGS. 3 and 4, the problem is evident: when the analog audio signal with its input level exceeding the maximum modulation factor of 50 percent is converted to a one-bit digital audio signal, the multi-bit digital audio signal DM acquired by the decimation filter 55 from the one-bit digital audio signal is clipped to a maximum level (full bit level) and manifests an extremely degraded distortion factor, although that is not the case with the analog audio signal A0 obtained by subjecting the one-bit digital audio signal simply to digital-analog conversion.
As described, there are cases where the decimation filter 55 with its maximum modulation degree determined beforehand is used to make up a converter that converts a one-bit digital audio signal through down-sampling into a multi-bit digital audio signal. The trouble is that when the level of an input one-bit digital audio signal exceeds the maximum modulation degree, the multi-bit digital audio signal is fixed to its clip level.
Once the signal level is clipped by the decimation filter 55, the clipped state cannot be resolved by any downstream device executing level attenuation of the multi-bit signal. Furthermore, it is difficult theoretically to vary the level of the one-bit signal as desired. Attempts to vary the one-bit signal level would require expanding the scale of necessary signal processing circuits.
In particular, in the case of applications involving the one-bit digital audio signal system represented by the Super Audio CD, the maximum modulation degree is basically defined in terms of recording signals onto discs. There is a possibility that, reflecting the recording party""s intentions, a disc may be produced with overmodulation, i.e., having levels exceeding the maximum modulation degree. When reproducing signals from such a disc, a disc player may subject a reproduced one-bit digital audio signal simply to digital-analog conversion for analog signal output. If the disc player has analog circuits with a sufficient dynamic range margin, those portions of the disc which are at an overmodulated level are played back at the same level as that in effect during recording.
When a decimation filter with its predetermined maximum modulation degree is used to convert a one-bit digital audio signal through down-sampling into a multi-bit audio signal that is output to a digital audio interface, the signal portions at overmodulated levels are always clipped to the maximum level during reproduction. In such a case, there are differences in terms of level and distortion factor between the analog and the digital output.
It is therefore an object of the present invention to overcome the above and other deficiencies and disadvantages of the prior art and to provide a transmitting apparatus and a reproducing apparatus wherein a one-bit digital audio signal is converted by a decimation filter constituting part of a converter into a multi-bit digital audio signal through down-sampling, and wherein the input of a one-bit digital audio signal exceeding a maximum modulation degree is prevented from resulting in a multi-bit digital audio signal with its level clipped to a full bit level (maximum level) while an average level of the output signal is improved.
In carrying out the invention and according to one aspect thereof, there is provided a transmitting apparatus comprising: attenuating means for attenuating by a predetermined quantity a first digital signal which is sampled at a first sampling frequency and which has one quantization bit; converting means for converting the first digital signal which was attenuated by the attenuating means, which was sampled at the first sampling frequency and which has one quantization bit, into a second digital signal which is sampled at a second sampling frequency lower than the first sampling frequency and which has multiple quantization bits; level detecting means for detecting a level of the second digital signal which was converted by the converting means, which was sampled at the second sampling frequency lower than the first sampling frequency and which has multiple quantization bits; and amplifying means for amplifying the level of the second digital signal which was sampled at the second sampling frequency and which has multiple quantization bits, in accordance with the level detected by the level detecting means.
According to another aspect of the invention, there is provided a reproducing apparatus comprising: reproducing means for reproducing from a recording medium a first digital signal which is sampled at a first sampling frequency and which has one quantization bit; attenuating means for attenuating by a predetermined quantity the first digital signal reproduced by the reproducing means; converting means for converting the first digital signal attenuated by the attenuating means into a second digital signal which is sampled at a sampling frequency lower than the first sampling frequency and which has multiple quantization bits; level detecting means for detecting a level of the second digital signal converted by the converting means; amplifying means for amplifying the level of the second digital signal which was sampled at the second sampling frequency and which has multiple quantization bits, in accordance with the level detected by the level detecting means; and transmitting means for transmitting the second digital signal amplified by the amplifying means.